Compensating system for a hexapod

ABSTRACT

A hexapod, for example, a hexapod used to position a tool with respect to a part to be machined, includes two spaced plates interconnected by six articulated and extensible legs. The two plates, including a base plate and a mobile plate, and the six legs are joined for varying, at will, the relative position of the mobile plate with respect to that of the base plate. The hexapod further includes a device for reducing and/or compensating for stresses on the elements of the hexapod, which can be a mechanical device, such as a jack, coupling a fixed surface with the periphery of the mobile plate and/or logic (in software) for defining at least one force-limiting criterion.

BACKGROUND OF THE INVENTION

The present invention concerns an improvement for a hexapod.

A hexapod can, for example, be used as a sub-assembly of a toolingmachine used for positioning a tool with respect to a part to bemachined. Hexapods can also be used in other applications, for example,to perform physical measurements.

A hexapod includes two plates, a working plate and a base plate, whichare interconnected by six legs. The legs are articulated and extensiblefor varying, at will, the relative position of the working plate withrespect to that of the base plate. Positioning and movement of the legsof the hexapod are controlled by a computer. An example of a hexapod ofthis type is described in EP 0 489 857.

SUMMARY OF THE INVENTION

It has been found to be desirable to provide a device that can reduce orcompensate for stresses on the assembly of elements comprising thehexapod, particularly the legs and articulating parts of the hexapod.

For example, a hexapod for positioning a tool with respect to a part tobe machined includes two plates, a mobile plate and a base plate, whichare interconnected by six articulated and extensible legs for varying,at will, the relative position of the mobile plate with respect to thatof the base plate. In accordance with the present invention, the hexapodis provided with a device for reducing or compensating for stresses onat least one of the elements of the hexapod.

To this end, a jack is provided which is received by a fixed surface andwhich operates to support the weight placed on the mobile plate of thehexapod. The jack can be placed vertically, including a rod which isfixed to the periphery of the mobile plate. As an alternative, the jackcan be non-vertically positioned, and the compensating forces can betransmitted by a return pulley to an attachment point on the mobileplate. As a further alternative, the jack can control a movable armwhich is then connected to the mobile plate.

Stresses on elements of the hexapod can also be reduced or compensatedfor by controlling the logic (in software) which is used to define atleast one force-limiting criterion for operation of the hexapod. It isalso possible to combine a mechanical compensation device with controlof the logic (in software) for defining force-limiting criteria foroperating the hexapod.

The present invention is further described below, with reference to thefollowing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustration of a first alternative embodiment ofa stress compensating device produced in accordance with the presentinvention applied to a hexapod having a main axis which is approximatelyhorizontal.

FIGS. 1B and 1C are schematic illustrations of further alternativeembodiments of the device of the present invention.

FIG. 2 is a schematic illustration showing possible movements of themobile plate of a hexapod.

FIGS. 3A, 3B and 3C are schematic illustrations showing alternatives forjoining the compensating device and the mobile plate.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A schematically illustrates a hexapod (1) which can be used, forexample, to position a tool with respect to a part to be machined. Thehexapod (1) includes two plates, a mobile plate (2) and a base plate(7), which are interconnected by six articulated and extensible legs(8). The legs (8) can be articulated and extended, at will, to vary therelative position of the mobile plate (2) with respect to that of thebase plate (7). In accordance with the present invention, the stresseson at least one of the components of the hexapod (1) are reduced orcompensated for as follows.

One way to achieve this is to act on the logic for accessing a point onthe trajectory of the tool.

Generally speaking, a hexapod has six degrees of freedom. However, onlyfive of these are used. In effect, the arc of rotation of the electropinis most often not used. There is, therefore, an infinite leg length forreaching a point on the trajectory of the tool.

In this regard, leg lengths are selected by accessing logic, controlledby a computer, which defines criteria that can be taken in isolation orin series. This can include logic for defining criteria such asremaining within the limits of the travel of each leg, remaining withinthe limits of angulations, limiting forces, obtaining desired rigidity,obtaining desired precision, and keeping a constant orientation aroundthe pin axis.

One way to reduce or compensate for stresses on at least one of thecomponents of the hexapod (1) is to control, with the computer, at leastone of the criteria for accessing a point on the trajectory. Such asolution is purely computer-oriented.

Another way to reduce or compensate for stresses on at least one of thecomponents of the hexapod (1) is to use a mechanical compensationdevice. Examples of such mechanical compensation devices are describedwith reference to FIGS. 1A, 1B and 1C.

In FIGS. 1A, 1B and 1C, the hexapod (1) is placed horizontally, and isequipped with a mechanical compensation device implemented with ahydraulic or pneumatic jack (S₁). In each example, the jack (S₁)provides constant power, and supports the weight placed on the mobileplate (2). Although the hexapod (1) illustrated in these figures hasbeen placed approximately horizontally, it is also possible for thehexapod to be placed in a vertical orientation.

In a first embodiment, shown in FIG. 1A, the jack (S₁) is placedvertically, coplanar with the mobile plate (2). The reference numbers 3and 4 respectively denote the axis of the pin and the main horizontalaxis of the hexapod (1).

The body of the jack (S₁) is supported by a fixed frame (6) and the rodof the jack (S₁) engages the mobile plate (2). The rod is fixed to theperiphery of the terminal member, the mobile plate (2) of the hexapod(1), to avoid the risks of collision between the two structures.

The travel of the jack (S₁) is sufficient to accept all variations inthe situation of the mobile plate (2), including both its position andorientation. It is important to have regular air flows available (bysuitably adjusting the orifices of the jack, and the sizes of thedistributor and the pressure regulator) so that variations in the volumeof the chamber of the jack do not result in additional loads.

FIG. 1B shows a second embodiment for the jack (S₁). In this embodiment,unlike the embodiment shown in FIG. 1A, the jack (S₁) is not placedvertically. Instead, compensating forces are transmitted by a cable (9)which passes over a return pulley (10) and which is connected to anattachment point on the mobile plate (2).

FIG. 1C shows a third embodiment for the jack (S₁). In this embodiment,the rod extending from the jack (S₁) controls a movable arm (5) which isappropriately connected to the mobile plate (2).

The compensating forces of the jack (S₁) make it possible to re-centerthe working ranges of each leg of the hexapod (1) to load ranges thatcan be accepted by the electric activators, and are able to reduce theforces of extreme loads. This also has the advantages of simplifying thetechnology to be implemented, taking advantage of the tested reliabilityof pneumatic jacks, and allowing the compensating support to be used toassist in the assembly and disassembly of the head of the hexapod on themachining center.

To simplify the hanging of the compensation device, it is possible tohave the mobile plate (2) maintain two parallel axes, eliminatingrotation about the axis of the pin, or to provide a compensation devicewhich can support all inclinations of the mobile plate (2). Referring toFIG. 2, inclinations of the mobile plate (2) result from movementsincluding rotation (R₁) around the pin, rotation (R₂) about a horizontalaxis, and tilting (B) in a vertical plane.

Testing has been done to develop standards for the support, to determinethe preferred application point for the compensating forces of the jack,and to determine the optimum compensating force of the jack. As anon-restrictive example, for a jack with a travel of 800 mm, an optimumcompensating force of 3500 N at a point situated on the periphery of themobile plate (2) and vertically offset by 6 mm (i.e., along the z axisof a Cartesian (x,y,z) system associated with the mobile plate) would beselected.

It is also possible to reduce or compensate for stresses on at least oneof the components of the hexapod (1) by combining computer compensationof the accessing logic with a mechanical compensation device, forexample, one of the previously described mechanical compensationdevices.

To facilitate movement of the plate (2) which is supported by theselected mechanical compensation device, it is possible to equip thehexapod with a link joined with the compensation device. For example, anarticulated link can be provided at the point of rotation (11) aroundthe pin, as is shown in FIG. 3A, or at the point (12, 12′) where themobile plate rotates about an axis, as is shown in FIGS. 3B and 3C,respectively. These three articulations can also be combined.

To improve control, the gain for the automatic action to be developedcan be controlled according to the leg length and/or speed. Thepressures in the mechanical compensation device can also be controlledaccording to these factors.

What is claimed is:
 1. A hexapod used to position a tool with respect toa part to be machined, comprising: two spaced plates, including a baseplate and a mobile plate, interconnected by six articulated andextensible legs, wherein the spaced plates and the legs are joined forvarying the position of the mobile plate relative to the position of thebase plate; and a jack connected between a fixed frame and the mobileplate, wherein the fixed frame is separate from the hexapod, wherein thejack supports weight placed on the mobile plate, for reducing orcompensating for stresses on at least one element of the hexapod;wherein the tool is placed on an axis normal to the mobile plate and thebase plate which is substantially horizontally oriented; and wherein thejack is connected to the mobile plate by a connecting structure which issubstantially vertically oriented, to suspend the mobile plate and theweight placed on the mobile plate from the fixed frame.
 2. The hexapodof claim 1 wherein the fixed frame is a fixed surface adjacent to thehexapod.
 3. The hexapod of claim 1 wherein the jack is substantiallyvertically oriented, and wherein the jack is connected to peripheralportions of the mobile plate by a connecting rod.
 4. The hexapod ofclaim 3 wherein the connecting rod is substantially vertically oriented.5. The hexapod of claim 1 wherein the jack is non-vertically oriented,and wherein the jack is connected to peripheral portions of the mobileplate by a cable extending over a return pulley, for engaging the mobileplate.
 6. The hexapod of claim 5 wherein portions of the cable extendingbetween the jack and the return pulley are non-vertically oriented, andwherein portions of the cable extending between the return pulley andthe peripheral portions of the mobile plate are substantially verticallyoriented.
 7. The hexapod of claim 1 wherein the jack is connectedbetween the fixed frame and a movable arm which is connected to themobile plate.
 8. The hexapod of claim 7 wherein the jack isnon-vertically oriented, and wherein the movable arm is connected toperipheral portions of the mobile plate by the connecting structurewhich is substantially vertically oriented.
 9. The hexapod of claim 1wherein the jack exerts a compensating force at a location which isoffset from a center axis of the hexapod which receives the tool andwhich is normal to the mobile plate and the base plate.
 10. The hexapodof claim 9 wherein the center axis of the hexapod is substantiallyhorizontally oriented, and wherein the jack exerts the compensatingforce at a location which is vertically offset from the center axis ofthe hexapod.
 11. The hexapod of claim 1 wherein movement of the mobileplate relative to the base plate is controlled by logic associated witha controller, and wherein the logic defines at least one force-limitingcriterion.
 12. The hexapod of claim 1 which further includes at leastone articulated link connecting the jack with the mobile plate.